Ten children with either Sillence type III or IV Osteogenesis Imperfecta have participated in a randomized prospective study evaluating the discontinuation of long leg braces. Each patient was paired with a partner determined to have similar clinical characteristics. Within each of 5 pairs, one child received braces for 16 months and the other did not. Then the assignment was switched. The children were evaluated every four months at NIH. At each admission they received the following: measures of height, weight, bone density (vertebral body, femur, radius), x-rays of long bones of lower extremity and spine, muscle strength testing (MMT)videobased gait analysis, range of motion (ROM). In addition, we have received monthly information completed by parents, describing each child's activities or a two week period. A clinical severity rating was performed by 2 clinicians at each NIH visit. Data show that there was no significant carryover effect from being braced first. Implying the braced and unbraced intervals could be compared. Muscle strength declined during the study in both braced and unbraced intervals. The latter showed a more substantial decline. The quadriceps was most affected. Clinical severity scores, a functional assessment, were higher during the unbraced interval; implying that children were more functional during the braced intervals (p=.02). Children were participating in more activity at the moderately active level of intensity while in braces than when unbraced; and less minimal activity (ie: less sedentary and more upright activity) ( p=.02). More lower extremity fractures occurred during the unbraced than braced intervals; but this was not statistically significant. (p=.21)